It is known that metals such as titanium, titanium alloys, nickel-based super alloys, stainless steel and lo-alloy steel exhibit an intensive affinity towards oxygen and nitrogen particularly when in a molten state. In fact, titanium shows such an extreme affinity to oxygen that it is oftentimes employed as an oxygen "getter." As such, when melting such metals and metal alloys for casting purposes and otherwise, it is necessary to provide a costly and elaborate vacuum system as an adjunct to an arc prelection beam furnace to prevent the pick up of contaminating gases.
It has been taught that molten metal could be continuously cast from a ladle into an ingot mold while shielding the molten metal with a liquified inert gas such as nitrogen (when the presence of this element in the metal is not harmful) or liquid argon. This is taught, for example, in British Patent No. 987190 which also teaches the use of an inert gas to shield the surface of molten metal in a ladle to avoid oxygen, hydrogen and nitrogen pick up from the surrounding atmosphere.
In electrical furnaces, molten metal comes from the heating of pieces of metal or of scrap metal which are progressively melted in the furnace while new pieces of metal or scrap are added throughout the melting phase.
It is known that almost any open surface of molten metal can be protected against oxygen, hydrogen or nitrogen pick up by blanketing with liquid argon, nitrogen or carbon dioxide snow. By incorporating inert gases at or above the surface of the molten metal, atmospheric oxygen and humidity-generating hydrogen can be purged/displaced from the surface of the melt to prevent contamination. The prior art has also considered protecting pieces of scrap metal or new stocks of metal in the stage of pre-heating above the liquid bath of molten metal prior to melting. The atmosphere above the metal is selected according to the nature of metals, alloyed metals, alloys or pure metals and is maintained above and around each element of the charge throughout the whole melting and holding operations from the very moment the charge begins to heat up to the moment the metal is tapped. Also dealing with metals which do not exhibit such a strong oxygen affinity, the practice of covering the surface of molten metal with liquid argon has been carried out and maintained until the metal is poured. However, metals, such as titanium, exhibit such a strong affinity toward oxygen that pouring such metals into a mold would negate the beneficial effects achieved in creating an inert atmosphere above the melt.
As noted in U.S. Pat. No. 4,806,156, attempts to maintain residual oxygen in a vessel at levels below 1% have not been entirely successful. This is particularly true when the level of molten metal in the furnace reaches about two-thirds of the height of the furnace. Oxygen concentration stabilizes at about 3-5% (volume concentration) at this height utilizing the foregoing patent teaching. Though still being considered as good protection as the atmosphere is approximately 20.9% oxygen, it is not completely satisfactory. This is obviously an acute problem in dealing with high oxygen affinity metals and their alloys such as titanium where even small quantities of oxygen can contaminate the charge and result in defects and anomalies which would translate into an unacceptable metal part or component.
When producing high grade metals and alloys of titanium and of other high affinity metals, for example, for the aeronautic industry, standard vacuum techniques are employed. When low grade metal and alloys are produced it is commonplace to engage in no protective measures to prevent oxygen contamination. There is currently a need to produce metals and alloys of high grade, but at a lower grade than that demanded by the aircraft industry, but at a much lower cost, which production under air cannot achieve.
It is thus an object of the present invention to provide a process for casting high oxygen affinity metals or metal alloys while avoiding the need to provide costly and complex vacuum systems.
It is a further object of the present invention to provide a process for casting high oxygen affinity metals while substantially preventing the pick up of contaminating gas during melting in an induction furnace.